Should optimizing compilers automatically assign registers?

[Istvan Mohos]

main() { /* prog1, compile with: /bin/cc -O prog1.c -o reg */
    static unsigned char buf[4096];
    register unsigned char *p;
    for (p = buf + 4096; --p >= buf;)
        for (; *p < 255; ++*p);
}

main() { /* prog2, compile with: /bin/cc -O prog2.c -o nonreg */
    static unsigned char buf[4096];
    unsigned char *p;
    for (p = buf + 4096; --p >= buf;)
        for (; *p < 255; ++*p);
}

Prog1 and prog2 are identical except that prog1 requests the
assignment of the auto variable p to a hardware register.
I mistakenly assumed that by using the -O flag, the compiler
would optimize prog2, and generate identical objects from
prog1 and prog2.  Of the systems I tried this on, the Pyramid90X,
the Sparcstation1, and a SystemV 6300+ PC, proved this to be true.
A VAX Ultrix and a Sun3 proved me wrong.  Here are some execution
times for the reg and nonreg objects, gotten on quiescent systems
using
   /bin/time ./[non]reg >& [non]regtime

Ultrix:
regtime         15.8 real         3.1 user         0.9 sys  
nonregtime      21.6 real         3.7 user         1.2 sys  

6300+:           7.9 real         7.8 user         0.1 sys  

Pyramid:         4.8 real         4.6 user         0.1 sys  

Sun3:
regtime          1.7 real         1.6 user         0.0 sys  
nonregtime       2.2 real         2.2 user         0.0 sys  

Sparcstation1:   0.9 real         0.8 user         0.0 sys  

Is the automatic allocation of a few variables into registers
too much to ask from an optimizing compiler?
-- 
        Istvan Mohos
        ...uunet!pyrdc!pyrnj!hhb!istvan
        RACAL-REDAC/HHB 1000 Wyckoff Ave. Mahwah NJ 07430 201-848-8000
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